1. Field of the Invention
The present invention relates generally to an accurate measurement of a part, and more specifically to a coordinate measurement machine with improved speed of operation.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A CMM (coordinate measurement machine or center measurement machine) is an apparatus used to inspect features that require tight control in the order of 0.0001 inch. To use a CMM, a programmer must create an inspection program for the CNN to accurately and correctly probe the part relative to an absolute point. The output of the CMM is X,Y,Z coordinates. These coordinates can be post-processed to create lines, planes and surfaces. The CMM apparatus includes a probe that is used to determine a coordinate by touching the probe tip to the part that is to be measured.
The coordinate measuring machine (CMM) is a device for measuring the physical geometrical characteristics of an object. This machine may be manually controlled by an operator or it may be computer controlled. Measurements are defined by a probe attached to the third moving axis of this machine. Probes may be mechanical, optical, laser, or white light, among others. The typical CMM is composed of three axes, an X, Y and Z axis. These axes are orthogonal to each other in a typical three dimensional coordinate system. Each axis has a scale system that indicates the location of that axis. The machine will read the input from the probe as directed by the operator or programmer. The machine then uses the X,Y,Z coordinates of each of these points to determine size and position.
A coordinate measuring machine (CMM) is also a device used in manufacturing and assembly processes to test a part or assembly against the design intent. By precisely recording the X, Y, and Z coordinates of the target, points are generated which can then be analyzed via regression algorithms for the construction of features. These points are collected by using a probe that is positioned manually by an operator or automatically via Direct Computer Control (DCC).
The CMM program must be tested to ensure the CMM probe does not run into the part or apply too much pressure because the part is not where the CMM program expects it to be. This process is very slow and does not work well with parts that have a large variation from one to another. Thus, the movement of the probe must be kept at a very low speed so as not to damage the probe. Therefore, the process of accurately measuring the part will take a long time.
In the early days of coordinate measurement, mechanical probes were fitted into a special holder on the end of the quill. A very common probe was made by soldering a hard ball to the end of a shaft. This was ideal for measuring a whole range of flat, cylindrical or spherical surfaces. Other probes were ground to specific shapes, for example a quadrant, to enable measurement of special features. These probes were physically held against the work piece with the position in space being read from a 3-Axis digital readout (DRO) or, in more advanced systems, being logged into a computer by means of a footswitch or similar device. Measurements taken by this contact method were often unreliable as machines were moved by hand and each machine operator applied different amounts of pressure on the probe or adopted differing techniques for the measurement.
A further development was the addition of motors for driving each axis. Operators no longer had to physically touch the machine but could drive each axis using a hand box with joysticks in much the same way as with modern remote controlled cars. Measurement accuracy and precision improved dramatically with the invention of the electronic touch trigger probe. Although still a contact device, the probe had a spring loaded steel ball (later ruby ball) stylus. As the probe touched the surface of the component, the stylus deflected and simultaneously sent the X,Y,Z coordinate information to the computer. Measurement errors caused by individual operators became fewer and the stage was set for the introduction of CNC operations and the coming of age of CMMs.
Optical probes are lens-CCD-systems, which are moved like the mechanical ones, and are aimed at the point of interest, instead of touching the material. The captured image of the surface will be enclosed in the borders of a measuring window, until the residue is adequate to contrast between black and white zones. The dividing curve can be calculated to a point, which is the wanted measuring point in space. The horizontal information on the CCD is 2D (XY) and the vertical position is the position of the complete probing system on the stand Z-drive (or other device component). This allows entire 3D-probing.
There are newer models that have probes that drag along the surface of the part taking points at specified intervals. This method of CMM inspection is more accurate than the conventional touch-probe method and most times faster as well. The next generation of scanning, known as laser scanning, is advancing very quickly. This method uses laser beams that are projected against the surface of the part. Many thousands of points can then be taken and used to not only check size and position, but to create a 3D image of the part as well. This “point-cloud data” can then be transferred to CAD software to create a working 3D model of the part. The laser scanner is often used to facilitate the “reverse engineering” process. This is the process of taking an existing part, measuring it to determine its size, and creating engineering drawings from these measurements. This is most often necessary in cases where engineering drawings may no longer exist or are unavailable for the particular part that needs replacement.
Optical probes and/or laser probes can be used (if possible in combination), which change CMMs to measuring microscopes or multi sensor measuring machines. Fringe projection systems, theodolite triangulation systems or laser distant and triangulation systems are not called measuring machines, but the measuring result is the same: a space point. Laser probes are used to detect the distance between the surface and the reference point on the end of the kinematic chain (i.e.: end of the Z-drive component). This can use an interferometrical, a light deflection or half beam shadowing principle.
White light inspection is a relatively new inspection process that uses optics to scan a surface. This process is quick and produces surfaces that can be compared to a solid model. A real part, such as a turbine rotor blade, can be scanned using a white light scanner to produce a computer generated 3D solid model of the part with details that include small cracks on the surface. The resulting computer generated solid model of the part can then be used in a FEA (finite element analysis) program for further modeling and testing. Unfortunately, the speed and convenience of white light scanning comes at the cost of accuracy. White light scanning has an order to magnitude more error (in the order of 0.001 inch) than a CMM.
White light scanning is the art, science, and technology of obtaining reliable information about physical objects through the process of recording, measuring, and interpreting photographic images. White light scanning uses the image of a 3D scene on a 2D medium to reconstruct a reliable and accurate model of the original 3D scene. As a non-contact measuring device, the ATOS III system retrieves all off its data through photographs. The system is comprised off a projector and two cameras which are mounted to a frame. By projecting a fringe pattern onto an object and simultaneously recording the pattern, the software is able to create an accurate point cloud of up to 4 million points of the surface of the object. For larger projects, reference points are added which allow a quick and accurate way to join together multiple measurements. The ATOS IIII system is able to take on projects from the size off a dime up to the size off an airplane. After the measurements are taken, the software processes the data creating a single point cloud file with a common point of origin. The data can then be aligned and manipulated to suit the needs of the required task.